Speaker Wire

This was a recommendation from Rock The Boat Marine Audio website about using doorbell wire for speaker wire. (If you are particularly concerned about corrosion, you can use 18-gauge doorbell wire or any other solid core wire.) Anyone ever try that? I've always used 16 gauge wire, is that a little overkill?

You don't want to use solid wire. Get some good 14-16 gauge speaker wire with as many strands as you can afford and then crimp or solder onto gold plated contacts (or waterproof connectors if possible).

I used 14 gauge stranded wire for my tower speakers. I can see using 16 gauge in a pinch but 18 seems too tiny.

This is for the 4 speakers in the cabin area and the amp is rated at 60rms x 4.

Do yourself a favor, do not use solid core wire for your installation. Use a 14 or 16 gauge as recommended above. As soon as you try and route the solid core wire the reason will be obvious why not to use it.